On the market today there are a lot of devices, particularly sunglasses, that have photosensitive light transmissivity. Some of these devices use chemical compounds that enter an excited state upon exposure to light--when in this excited state, they absorb more light. The primary drawback of this technology is that these compounds are slow to respond to a change in light conditions. For example, a sunglass wearer driving into a tunnel on a sunny day will have glasses that are too dark (too light absorbent) upon entry and too clear (too light transmissive) upon exiting the tunnel due to a slow change in light absorptivity. As a result, the wearer is put in an uncomfortable and potentially dangerous situation.
Sunglasses that use liquid crystals are also known. Typically, a liquid crystal cell is placed between two polarizing layers. The liquid crystal modifies the polarization state of the transversing light. The polarizers, not dyes, are used to absorb light. The only use of dyes in the prior art is non-functional, generally for tint, as exemplified by U.S. Pat. No. 4,968,127. At least two major drawbacks to this construction is, first, that transmittance of light depends on the viewing angle, and second, it is hard to implement on a plastic substrate. As is well known, polarizers significantly reduce the viewing angle of the cell. In addition, the transmittance of light is typically too small for use in low lighting conditions. In U.S. Pat. No. 5,015,086, for example, the highest transmission reported was 35%.
Therefore, there is a need for a device where the transmittance of light is constant at all viewing angles, where attenuation of transmitted light is continuously and rapidly controllable, and where the device can be implemented on plastic substrates.